1. Field
The present invention relates generally to a reversible recording medium, and more particularly to a reversible thermosensitive recording medium capable of forming and erasing images repeatedly by utilizing a reversible change in transparency or in color of a thermosensitive layer with temperature.
2. Discussion of Background
A variety of information recording media have been developed to meet the demand for increasing volume and diversity of information. A reversible thermosensitive recording medium has been attracting much attention recently for its capability of forming and erasing images which can be carried out repeatedly on demand. In addition, such image forming and erasing may be achieved at relatively low cost without complicated developing steps.
Reversible thermal recording media are known to be capable of forming and erasing images by reversibly changing optical properties between two states such as, for example, an opaque state (image formed) and a transparent state (image erased), depending on temperature.
As representative examples of the reversible thermosensitive recording materials, there have been disclosed several recording materials such as a material including a nematic liquid crystal polymer material having an isotropic phase transition point higher than their glass transition temperature, capable of being reversibly erased by the application of light or heat, as described in Japanese Laid-Open Patent Application (referred to as "JPA", hereinafter) 63-191673 (1988); a film composed of a thermotropic liquid crystal polymer material in the choresteric phase, capable of being reversibly erased by the application of heat to thereby display transparent reversible color images, as described in JPA 2-135418 (1990); a dispersion in a crystalline matrix of both an electron-donating coloring agent having a lacton ring and an electron-accepting color developer having a phenolic hydroxy group, capable of being colored and erased reversibly by application of heat, as described in JPA 63-315288 (1988); a composition of both a leuco compound and its coloring or decoloring agent, capable of being colored or erased upon their reaction, as described in JPA 2-188293 (1990); a dispersion of low molecular weight organic materials such as higher alcohols or higher fatty acids which are included in a matrix resin such as polyester, capable of changing transparency reversibly depending on temperature to thereby record and erase information data, as described in JPA 54-119377 (1990) and 55-154198 (1991).
The known reversible thermosensitive recording media, however, have a shortcoming that portions of the recording layer may stick to a heating element, such as a thermal printhead, due to the large amount of friction therebetween. This causes the surface of the recording layer to become rough and to have approximately periodic features which are formed corresponding to the dot density of the thermal printhead. This results from the relatively easily deformable property of the layer and increases with the number of times of repeated heat application, thereby resulting in deterioration of the quality of images formed on the reversible thermosensitive recording medium.
In order to obviate the above-mentioned problem, an overcoat layer is proposed in JPA 62-55650 (1987), which is disposed on a recording layer to thereby reduce the coefficient of friction of the recording medium.
The above-mentioned overcoat layer, however, is found to have drawbacks such as an unsatisfactory adhesion to the underlying recording layer, resulting in peeling caused by repeated application of mechanical stress during the imaging processes, and in reduced quality of recorded images.
Further, to improve adhesion a layered structure for the recording medium is disclosed in JPA-1-133781 (1989), in which an intermediate layer composed primarily of resin is formed on a reversible thermosensitive recording layer, and an overcoat layer of heat-resistant resin is formed further thereon. By this expedient, the adhesion is found to be improved by the intermediate layer, and the deformation of the surface of the recording media is reduced by the overcoat layer.
In this case, however, there still remain disadvantages such as when scratches formed on the recording layer and portions of the overcoat layer are removed therefrom and then stuck to heating elements of a thermal printhead, by repeated application of mechanical stress during imaging processes. As these portions accumulate on the heating elements the heat conduction is hindered between the heating elements and the recording media, thereby resulting in difficulty in forming satisfactory images.
Still further, to obviate these disadvantages, a reversible thermosensitive recording medium is proposed in JPA-9-142037 (1997), in which an overcoat layer is provided on a recording layer for a reversible thermosensitive recording medium such that neither a peak temperature in the tan .delta. vs, temperature curve nor a characteristic temperature of dynamic relaxation related to the tan .delta. peak temperature is exhibited, wherein tan .delta. is defined by G'/G", with dynamic modulus of elasticity represented by G', and dynamic elastic loss represented by G".
By the use of the thus prepared overcoat layer, the above-mentioned roughness on the recording layer, which has approximately periodic features corresponding to the dot density of the thermal printhead, can be prevented to some extent.
These recording materials are, however, found to have drawbacks such as the occurrence of cracks and sticking. The cracks are formed on the overcoat layer more often for recent imaging processes because of the ever increasing speed, in which an image forming/erasure apparatus such as, for example, a thermal printhead is used for heating under more severe conditions such as (1) a shorter heating period of the order of several milliseconds in order to achieve faster image processing, and/or (2) an increased printing pressure in order to achieve clearer printed images, whereby visibility of the formed images deteriorates due to the decreased transparency caused by these cracks. Furthermore, with an increasing the number of cracks, the surface property of the overcoat layer deteriorates, with portions of the overcoat layer being removed and then sticking onto a heating element of the thermal printhead. This also deteriorates image quality.